1. Field of the Invention
The present invention relates to the measurement of temperature and particularly to the sensing of the temperature at a plurality of points from a single location located remotely from such points. More specifically, this invention is directed to a system wherein multiple temperature responsive "probes" situated at different points of interest may be individually interrogated and especially to a system wherein the "probes" are passive and intregal with a cable. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The sensing of temperature at multiple sites from a single location, a control room for example, is necessary or desirable in many fields of technology. Thus, in environments such as nuclear reactors, chemical processing facilities, computer installations, underground mines, storage facilities for various materials, etc. it is often necessary to monitor the temperature at a plurality of points in order to protect personnel and/or equipment from injury. The actual sensing of temperature at a site of interest can easily be accomplished using various commercially available sensor devices which provide an electrical signal commensurate with the local ambient temperature. However, in order to transmit the signals commensurate with the sensed temperature to a central monitoring location, it has been the customary practice to employ a hard wired system including a wire pair associated with each individual sensor device. At the monitoring location, i.e., at their receiving ends, these wire pairs are multiplexed to the measurement apparatus. This approach obviously requires a large number of wires. Further, since signal levels on the order of microvolts are generated by the sensor devices, very reliable contact to the sensor associated wires is required and sophisticated electronics must be employed to insure that the signals of interest will not be masked by noise. As a result of the above-discussed and other complexities of prior art hard wired systems, wherein a separate wire pair is associated with each temperature sensor, these systems have necessarily been lacking in the requisite reliability and have required constant maintenance.
It has also been proposed to transmit temperature related information from multiple remote measuring sites via optical fiber bundles. This, however, has proven to be prohibitively expensive for many applications due, in part, to the need to utilize active components at the measurement sites to transduce the measured temperature into an optical signal.
A thermocouple-type cable comprising a pair of dissimilar thermocouple wires separated by a ceramic such as magnesium dioxide has also been proposed. In such a cable a "hot spot" at any point along the cable will result in the generation of a temperature representative thermoelectric voltage. This voltage can be directly measured and translated into temperature. Impedance measurements can be made to determine the location of "hot spot" relative to a reference point. While such a thermocouple-type cable would eliminate the requirement for multiple wire pairs, tests have shown that reliable information cannot be obtained at temperatures below about 250.degree. C. For most potential applications, the temperature induced injury which is to be avoided will occur below 250.degree. C.